Remove obsolete check for GCC < 3 and support for Intel Compiler

We no longer support old versions of GCC. Remove this check by
assuming it's false. That will make the entire expression false. Also
remove support for Intel compiler, it's badly bitrotted. Technically,
this removes support for C89 and K&R from compilers that don't define
_Bool in those compilation environments as well. I'm unaware of any
working compiler today for which that would be relevant (pcc has it
and tcc sadly isn't working for other reasons), though if one
pops up in ports, I'll work to resolve the issue.